Divergent self-assembly propensity of enantiomeric phenylalanine amphiphiles that undergo pH-induced nanofiber-to-nanoglobule conversion.

Abstract

This study presents the pathway diversity in the self-assembly of enantiomeric single phenylalanine derived amphiphiles (single F-PDAs), <i>viz.</i>L-NapF-EDA and D-NapF-EDA, that form supramolecular hydrogels at varied concentrations (≥1 mg mL^-1 and ≥3 mg mL^-1, respectively). By fitting the variable temperature circular dichroism (VT-CD) data to the isodesmic model, various thermodynamic parameters associated with their self-assembly, such as association constant (<i>K</i>), changes in enthalpy (Δ<i>H</i>), entropy (Δ<i>S</i>), and Gibbs free energy (Δ<i>G</i>), were extracted. The self-assembly of these single F-PDAs was found to be enthalpy-driven but entropically-disfavored. Although self-assembly of the D-isomer was slow, it also exhibited greater free energy of association than the L-isomer. Consequently, thermally and mechanically more robust self-assemblies were formed by the D-isomer than the L-isomer. We term these results as the "butterfly effect in self-assembly" wherein the difference in the stereochemical orientation of the residues at a single chiral center present in these molecules resulted in strong differences in the self-assembly propensity as well as in their thermal and mechanical stability. These single F-PDAs form helical nanofibers of opposite chirality upon self-assembly at basic pH (≥8) that produce intense CD signals. However, upon decreasing the pH, a gradual nanofiber-to-nanoglobular transformation was noticed due to protonation-induced structural changes in the PDAs.